Ok, so I mostly understand how vowel devoicing works in Japanese, but there's something that's making me a bit confused. On all the sources I've found, most say that vowel devoicing usually occurs in a vowel sound in-between two voiceless consonants - specifically k, s, sh, t, ch, h, f and on rare occasions, apparently b, and p. However, there are a few times where I've seen a voiceless vowel occur with "d" as the second consonant, and at one point, I believe I've heard an "r" as the second consonant as well ( Examples: しだ、知らず ). Now, is it that maybe these are unique cases, or that I'm just not hearing them correctly? Or is there some other rule I'm missing?
The phonological rules of vowel devoicing in Japanese are as follows: high vowels (that is, in the case of Japanese, i and u), become unvoiced when surrounded by other unvoiced sounds. This is how it works on paper, and usually these rules are followed, but sometimes when people speak very quickly, these vowels become unvoiced even if one of the sounds surrounding it is voiced. It is also notable that typically males devoice vowel more frequently than females. Most of this information can also be found on Wikipedia: https://en.wikipedia.org/wiki/Japanese_phonology#Devoicing